Abstract ? Therapeutic Adjunct to Improve Immunosuppression Outcomes Following Organ Transplantation The goal of this project is to develop a safe, microbiome-targeted therapeutic to improve outcomes associated with mycophenolate (MPA), an immunosuppressant commonly prescribed for acute and long-term prevention of organ rejection after transplantation surgery. The therapeutic potential of MPA is frequently compromised by intolerable diarrhea that results in MPA dose modification in 30-40% of recipients in the first 6-12 months of therapy. MPA is the active moiety in two approved, widely used immunosuppressant drugs (mycophenolate mofetil and sodium mycophenolate). MPA blocks B and T cell activation by inhibiting inosine 5?-monophosphate dehydrogenase, an enzyme that is also important for growth and replication of enterocytes. MPA is detoxified by liver enzymes to the inactive phenolic MPA-glucuronide (MPAG). MPAG is excreted in bile and delivered to the intestinal lumen, where bacterial ?- glucuronidases (GUS) in the intestinal microbiota cleave MPAG back into its active MPA moiety. MPA produced in intestinal lumen is thought to exert several toxic effects to adjacent enterocytes. We hypothesize that MPA-induced diarrhea can be ameliorated by inhibiting the activity of bacterial GUS to reduce luminal production of toxic MPA. This proposal aims to validate this therapeutic hypothesis based on ex vivo and in vivo studies that collectively demonstrate: (1) Ex vivo protein preparations isolated from rat intestine effectively cleaves MPAG to MPA, and this cleavage can be reduced by novel and selective GUS inhibitors; (2) Diarrhea induced by MPA in rats can be alleviated with a proprietary bacterial GUS-targeted inhibitor; and (3) Pharmacologic inhibition of bacterial GUS can ameliorate MPA-induced diarrhea without significantly reducing systemic exposure of MPA. The proposed studies provide proof-of-concept that the microbiome can be selectively targeted to improve outcomes associated with MPA therapy. This Phase 1 project provides the groundwork for lead optimization and preclinical development activities in Phase 2.